Spray systems are well known in the paint industry for use in preparing painted test panels. Typically, a basecoat is sprayed onto the test panel and baked at a predetermined time and predetermined temperature to cure the paint. This is followed by applying a clearcoat which is sprayed over the basecoat and baked. The resultant panel is then compared against the standard color panel and if there is a color or other quality discrepancy, the batch of paint with which the panel was sprayed is modified accordingly. The modification process is known as shading. This time-consuming procedure, particularly for water-based basecoat and solvent-based clearcoat systems, of applying a basecoat, baking, then applying a clearcoat followed by baking, must be repeated for each shading iteration until the sample panel representing the batch color matches the color standard to a predetermined color tolerance range.
Traditionally, the shading process has been carried out by highly skilled and trained personnel who require extensive on-the-job experience to achieve proficiency in their craft. Since visual shading at best is an art, effective administration of the shading process was difficult.
In recent years, such visual shading has been supplemented by the use of apparatuses for instrumentally characterizing a paint or pigment composition. Colorimeters and spectrophotometers are well-known in the art and are used to measure the amount of light reflected at varying light wavelengths in the visible spectrum by a painted opaque panel that is held at a given angle relative to the direction of the incident light source. For nonmetallic paints, i.e., paints which do not contain any light-reflecting flakes or platelets, the reflectance factor has a minimum reflectance variation with the angle of the panel relative to the direction of incident light except at the gloss (specular) angle. Consequently, a single spectrophotometric reading at any specified angle will produce a reflectance value by which to accurately characterize the paint.
Metallic paints, on the other hand, contain light-reflecting flakes of such materials as aluminum, bronze, or coated mica. Such paints are characterized by "two-tone" or "flip-flop" effects whereby the apparent color of the paint changes at different viewing angles. This effect is due to the orientation of the flakes in the paint film. Since the color of such metallic paints will vary as a function of the angle of illumination and viewing, a single spectrophotometric reading is inadequate to accurately characterize the paint. Typically, three multiangular measurements are used to derive color constants for metallic paints. See, for example, Alman U.S. Pat. No. 4,479,718, issued Oct. 30, 1984 to Alman.
The color of the paint is described in L*, a* and b* values which are coordinates in visual uniform color space and are related to X, Y & Z tristimulus values by the following equations which have been specified by the International Committee of Illumination:
L* defines the lightness axis PA1 Xo, Yo and Zo are the tristimulus values of the perfect white for a given illuminant; PA1 X, Y and Z are the tristimulus values for the color.
L*=116(Y/Yo).sup.166 -16 PA2 a* defines the red green axis PA2 a*=500[(X/Xo).sup.1/3 (Y/Yo).sup.1/3 ] PA2 b* defines the yellow blue axis PA2 b*=200[(Y/Yo).sup.1/3 -(Z/Zo).sup.1/3 ]
where
The color vectors for each of the colorants used to prepare the paint are provided and determined as discussed above. The color vector is the movement in color space, i.e., the change in the L*, a* and b* values caused by the addition of a unit amount of each colorant used. For example. ##EQU1## where C1, C2 and C3 are the concentration of pigment or tint used in the color.
The color technology used in the shading process is well known and is fully discussed in F. W. Billmeyer and M. Saltzman, Principles of Color Technology, John Wiley and Sons, New York, 2nd Edition, (1981). Of particular interest is an article by A. B. J. Rodrigues in Fifth International Conference in Organic Coatings Science and Technology Proceedings, Vol. 3, Advances in Organic Coatings and Technology Series. "Theory and Implementation of Modern Techniques of Color Conception, Matching and Control", p. 272-282, (1979).
The color measurement procedure can be automated through use of a computer. See, for example, Armstrong U.S. Pat. No. 3,690,771, issued Sep. 12, 1972, and Falcoff U.S. Pat. No. 4,403,866, issued Sep. 13, 1983.
The colorimeter used in the process is electrically connected to a computer and preferably determines the L*, a* and b* values of the paint being prepared and feeds these values back to the computer. The colorimeter views the paint through the visible light spectrum of 400-700 nanometers (nm) for example, at 20 nm increments and calculates the L*, a* and b* values for the paint based on this data. It is possible to feed process signals from the colorimeter generated by viewing the paint to the computer and have the computer determine the L*, a* and b* values.
The computer takes these L*, and a* and b* values and determines the difference between the L*, a* and b* values of the paint being prepared and tolerance values for the standard paint (.DELTA.L*, .DELTA.a* and .DELTA.b*). With the vector information of the colorants and the .DELTA.XL, .DELTA.Xa* and .DELTA.b*, the computer determines the amount of each of the colorants that is to be added to bring the paint within the tolerance values for the paint and activates the metering pumps which feed colorants into the mixing vessel. The above procedure, is repeated until the paint being prepared is with L*, a* and b* tolerance values of the paint.
Reiterative spraying and baking of test panels with basecoat and clearcoat layers is a time consuming process. Typically, 25-40 minutes are required to spray, flash and bake, typically at 220.degree. F., a waterborne basecoat. An additional hour is typically necessary to cool the test panel, spray, flash and bake, typically at 275.degree. F., the solvent-borne clearcoat over the basecoat. There can be as many as 3-4 iterations, particularly for metallic paints, before the shading process will bring the paint batch into the range of acceptability for color tolerances.
The present invention decreases the overall time required to step through multiple iterations of the color shading process to reach an acceptable tolerance level for the batch of mixed paint, for both solid color and metallic paints.